I think this has to do with how ve.dm.ElementLinearData.isContentOffset() works. The article in case was Bayern-Kaserne@510852 (Parsoid HTML), with content like

<section> <table> <tbody> <tr> <th>...</th>...
         ^       ^       ^    ^     ^
         1       2       3    4     5

1, 2, 3 and 4 are what VE calls structural offsets - you can't insert text there, you need to insert some sort of structure first (in the case of 1 a <p> tag, for 2/3/4 a table caption / row / cell).. 5 is a content offset: you can add text and inline content such as an image. I assumed content offset always means <p> and so that's always the right place for an image (and hence the insertion logic finds the first such offset that's not between templates), but apparently table cells are content offsets as well.

I wonder if I got the assumption wrong? We are inserting an MWBlockImageNode, not an MWInlineImageNode, so maybe it should actually go to the structural offset instead? Or maybe it doesn't really matter and VE can figure out either way? Probably worth consulting the Editing team on this.

@MMiller_WMF we'd want to insert on top of the table, right? (Both table and image are right-floated so one will end up above the other.)

I filed another insertion breakage issue - T295922: [testwiki-wmf.9] Add image - small size image breaks caption box. I think that the issue happens because of a small size of the image, but I might be wrong about.

In T295815#7508643, @Tgr wrote:

@MMiller_WMF we'd want to insert on top of the table, right? (Both table and image are right-floated so one will end up above the other.)

@Tgr -- yes, we want the image to be above the table -- the spirit is that it should be the first thing in the article after the templates at the top (same as specifications here T290785: Add an image: edit business rules.

T295905: VE fails to place cursor when document starts with a table is somewhat related.

Change 740294 had a related patch set uploaded (by Gergő Tisza; author: Gergő Tisza):

[mediawiki/extensions/GrowthExperiments@master] [WIP] Add Image: Do not insert inside tables

https://gerrit.wikimedia.org/r/740294

Change 740294 merged by jenkins-bot:

[mediawiki/extensions/GrowthExperiments@master] Modify the insertion logic to insert before, not at, the first non-whitespace content offset, as that could be inside a table, list etc.

https://gerrit.wikimedia.org/r/740294

In T295815#7558230, @Etonkovidova wrote:

Accordingly, after adding an image, the position of the added image is not optimal:

That's the coordinate template being broken. Not much we can do about it & probably won't happen on real wikis.